Study of Photoexcited States and Nonlinear Optical Responses in Insulating Copper Oxides

Mott绝缘体中的电荷间隙来源于强电子关联。因此，我们预期光学响应不同于带状绝缘体中的光学响应。In.本项目利用半填充Hubbard模型及其有效模型，对典型的Mott绝缘体铜氧化物中的光激发态和非线性光学响应进行了理论研究。我们发现，与自旋和电荷分离的一维相比，光掺杂载流子和系统中局域自旋的相互作用在非线性光学响应中起着至关重要的作用。我们还发现二维绝缘铜氧化物中双磁子拉曼散射的反常入射光子能量依赖性是由自旋的存在引起的。为了阐明光激发态对维数的依赖性，我们研究了线性光吸收光谱的温度依赖性。在一维体系中，由于自旋-电荷分离的存在，在低温下没有明显的温度依赖性。相反，特征温度依赖性被发现在两个维度，这来自载流子和自旋的相互作用。非线性光学响应中不仅有光学允许态的贡献，而且有光学禁止态的贡献。为了研究这两种状态的分布，我们对这两种状态进行了动力学密度矩阵重整化群计算。一维哈伯德模型我们发现，在大库仑相互作用下所看到的状态的简并在真实的材料中被提升。

Charge gap in the Mott insulator originates from strong electron correlation. Therefore, we expect optical responses that are different from those in the band insulator. In. this project, we theoretically examine photoexcitaed states and nonlinear optical responses in insulating copper oxides that are typical Mott insulators by using a half filled Hubbard model and its effective modelWe investigated the nonlinear optical responses in the two-dimensional Mott insulator, applying the numerically exact diagonalization technique to the effective model. We found that, in contrast to one dimension where spin and charge are separated, the interplay of photo-doped carriers and localized spins in the system plays a crucial role in the nonlinear optical responses. We also found that anomalous incident-photon energy dependence of the two-magonon Raman scattering in two-dimensional insulating cuprates is caused by the presence of the spins. In order to clarify the dependence of photoexcited states on dimensionality, we examined the temperature dependence of the linear optical absorption spectrum. In one-dimensional system, the there is no remarkable temperature dependence at low temperature because of spin-charge separation. In contrast, characteristic temperature dependences are found in two dimensions, which come from the interplay of carriers and spins. Not only the optically allowed but also forbidden states contributes in the nonlinear optical responses. To examine the distribution of both the states, we performed a dynamical density-matrix renormalization group calculation for the. one-dimensional Hubbard model. We found that the degeneracy of the states seen at large Coulomb interaction is lifted in real materials.